A serious constraint in the increase of packing density of electronic integrated circuits is their thermal management; i.e., the ability to effectively carry away the heat generated by the electronic circuit elements (e.g. semiconductor chips and other components). The density of heat generating devices and their operating frequency (since the power dissipation of some types of circuits increases with frequency) both contribute to the problem of heat generation.
A heat sink can be incorporated within the circuit board or substrate material on which the circuit elements are mounted in some applications. The effectiveness of the heat sink increases with increasing thermal conductivity of the heat sink material. Diamond has the highest thermal conductivity (k=2000 W/m degree K. at 300 degrees K.) of any known material. Silver, copper and aluminum (with k=430, 400 and 240, respectively, at 300 degrees K.) are among the best cheaper alternative heat sink materials, but are electrical conductors, requiring special electrical insulating steps if isolated conductors must be passed through the material. Also, silver and copper (which are better thermal conductors than aluminum) are much heavier per unit volume than diamond. A further advantage of diamond is that its thermal expansion coefficient is a better match to that of silicon than most other heat sinking materials. Diamond has been suggested and used as a heat sink material for electronic devices and circuits because of its superior thermal conductivity and insulating properties, but its practical use has been limited by its cost/benefit ratio in circuit applications where alternative heat sinking materials may be more readily provided in the necessary sizes and shapes, and where such alternate materials are operationally adequate.
The cost/benefit ratio for diamond in circuit applications has improved, and will improve further, with the advent of better techniques for the production of synthetic diamond. In the copending U.S. patent application Ser. No. 592,209, filed Oct. 3, 1990, assigned to the same assignee as the present Application, there is described a circuit, such as a computer processor, which utilizes a multiplicity of generally planar diamond substrate layers and a multiplicity of generally planar spacer boards formed of insulating material. Each of the substrate layers has mounted thereon a multiplicity of electronic circuit elements and conductive means for coupling between electronic circuit elements. ["Circuit elements" generally includes active as well as passive devices or components of any kind used in electronic or electro-optical applications.] Some or all of the electronic circuit elements on the substrate layers may be integrated circuit chips. The substrate layers and spacer boards are stacked in alternating fashion so that spacer boards are interleaved between adjacent substrate layers. Each of the spacer boards has a multiplicity of electrical conductors extending through its planar thickness to effect coupling between circuit elements on the substrate layers on its opposing sides. Also, each diamond substrate layer has a multiplicity of electrical conductors or "vias" passing through its thickness, the vias coupling conductors on opposite sides of the substrate.
In a circuit of the type described in the referenced copending U.S. Patent Application, it is necessary to provide the multiplicity of conductive vias which pass through so-called "via holes" from one surface of the diamond substrate layer to the other. As noted therein, the via holes can be laser drilled and edge treated, if necessary, to remove local graphitization of the diamond. The via holes can be metal filled using plating or conventional metal paste filling. Conductive epoxy filling could also be used, but may give less reliable, higher resistance contacts to the metallization layers on the top and bottom surfaces of the substrate.
Prior art techniques for filling via holes with conductive material have been found to suffer certain disadvantages when utilized in filling the via holes in diamond substrates. One problem is cracking of the diamond substrate due to differences in the coefficients of expansion of the diamond material and the filling metal. Another problem is shrinking of the metal upon cooling, whereupon the formed conductive via can fall out of the via hole or loosen therein.
It is among the objects of the present invention to provide solution to the described problem, and to set forth an improved technique for fabricating conductive via hole connections through a diamond substrate for utilization in an electronic circuit.